1. Technical Field
The present invention relates to a method for fabricating semiconductor devices and heat treatment apparatuses. More particularly, the invention relates to a technique that is suitable for heat treatment in the fabrication process of an active matrix substrate in which pixel electrodes are driven by thin film transistors (hereinafter referred to as TFTs).
2. Description of the Related Art
Substrates provided with TFTs include, for example, an active matrix substrate used in a liquid crystal device (liquid crystal panel) as an electro-optical device. In the active matrix substrate, pixel electrodes are arranged in a matrix on a non-alkaline glass substrate as an insulating substrate, a TFT formed of a polysilicon thin film or the like is connected to each pixel electrode, and the liquid crystal is driven by applying a voltage to each pixel electrode through the TFT.
When such an active matrix substrate is fabricated, a semiconductor thin film is formed on a non-alkaline glass substrate in a given pattern, and by using the semiconductor thin film, active elements such as TFTs and diodes or passive elements such as resistors and capacitors are formed. During the formation of the semiconductor thin film such as silicon (Si), since factors that inhibit electrical characteristics of the electrical elements (lattice defects, radiation damage, internal strain, etc.) occur in the Si film or the like, various types of heat treatment (annealing) are performed in order to decrease the defects.
In such annealing, by heating the Si film or the like having the defects as described above up to a relatively high temperature, defect repairs (for example, disappearance of atomic holes or shift of dislocation into a stable state), or activation of impurities (for example, an increase in proportions of the injection ions that function as donors or acceptors) are achieved.
When annealing as described above is performed, an annealing furnace is conventionally used (furnace annealing). However, in such furnace annealing, since extended periods of time (for example, several hours depending on the conditions) are required for treatment, by heating (at a temperature of 500.degree. C. or more, or 700.degree. C. or more), warpage may occur in glass substrates having poor heat resistance, or impurities may diffuse excessively in a semiconductor film, resulting in deterioration in device characteristics. Therefore, particularly, in a low temperature process in which low temperatures are required for treatment, it has been difficult to employ furnace annealing as an annealing method when TFTs or the like are formed from a polysilicon film.
Accordingly, recently, various types of low temperature annealing and rapid thermal annealing, which are applicable to the formation of TFTs or the like using polysilicon formed by a low temperature process were developed. Among low temperature annealing and rapid thermal annealing methods, a laser annealing method has been widely used as a heating means to crystallize silicon films and to activate impurities in the case of forming polysilicon TFTs by a low temperature process.
The laser annealing method is adopted, for example, as a heat treatment or the like, in which beams of an excimer laser using XeCl, KrF, or the like are focused linearly by an optical system, and by irradiating an amorphous silicon (a-Si: amorphous silicon) film or the like formed on a glass substrate with the linear beams, the amorphous silicon film is fused instantaneously and the amorphous silicon film is crystallized to a polysilicon (Poly-Si:polycrystalline silicon) film.
Conventionally, in a heat treatment apparatus such as a heat treatment furnace, a substrate is placed on a table composed of silica glass or the like disposed in the heat treatment furnace when annealing is performed.
However, in recent years, as there is strong demand for performance improvements such as high definition in liquid crystal panels and the like, the problem of thermal stresses in semiconductor thin films caused by laser annealing of active matrix substrates is appearing.
That is, in view of improvements in characteristics such as responsiveness of liquid crystal panels, further improvements in switching characteristics of TFTs (i.e., on-off characteristics of n-channels or p-channels) are expected, and the quality of crystallinity of polysilicon or the like included in TFTs is important for improvements in switching characteristics. For example, when an amorphous silicon film is deposited at a thickness of 500 angstroms on a non-alkaline glass substrate by a low pressure chemical vapor deposition (LPCVD) system, and then the amorphous silicon film is crystallized into a polysilicon film by irradiating the amorphous silicon film with an excimer laser and instantaneously heated, the silicon film is preferably crystallized into polycrystalline silicon at a rate of 90% or more. However, in the known laser annealing method, after the amorphous silicon film irradiated with an excimer laser and is heated instantaneously (e.g., for several tens of nanoseconds) up to approximately 1,000.degree. C., it is instantaneously cooled down to room temperature. Therefore, thermal stresses occur in the crystallized polysilicon film, and because of crystal defects or the like caused by the thermal stresses, polycrystalline silicon fail to achieve a sufficient crystallization rate.
The problem described above will be described in detail with reference to FIGS. 9 and 10. FIG. 9 is a graph which shows the analysis results of a normally crystallized polysilicon film by means of Raman spectroscopy. FIG. 10 is a graph which shows the analysis results of a polysilicon film, by means of Raman spectroscopy, which has been crystallized by heat treatment using a conventional laser annealing method.
As is clear from the drawings, the normally crystallized polysilicon (Poly-Si) has a Raman spectrum peak at a wavelength of 520.00 (cm.sup.-1), as shown in FIG. 9. In contrast, in the polysilicon film which has been crystallized by heat treatment using the conventional laser annealing method, the Raman spectrum peak is shifted from 520.00 (cm.sup.-1) to a shorter wavelength, as shown in FIG. 10. Presumably, since shrinkage occurs in the polysilicon film due to thermal stresses during laser annealing, resulting in defects in the polycrystalline silicon, the crystallization rate from the amorphous silicon to the polysilicon decreases. That is, it is believed that in the conventional laser annealing method, since high-density energy is locally applied for a short time, strains such as shrinkage occur in the polysilicon film during cooling.
In particular, in the laser annealing method used when an active matrix substrate is fabricated, as described above, excimer laser beams are focused linearly by an optical system for radiation, and in order to perform annealing of the entire substrate, the substrate is placed on, for example, a belt conveyor or roller, and is moved at predetermined timing in relation to the irradiation beams, or the beams are moved in relation to the substrate placed on a table. Accordingly, thermal stresses caused in the polysilicon film on the substrate also occur linearly along the irradiation beams. Therefore, linear stripes appear on the polysilicon film on the substrate, and warpage occurs in the entire substrate under the influence of the stresses generated linearly, which is greatly disadvantageous.
On the other hand, a lamp annealing method has been investigated as a low temperature annealing method or rapid thermal annealing method in which a halogen lamp is used instead of laser beams.
In the lamp annealing method, by irradiating a silicon film with energy beams for several seconds to several tens of seconds using a lamp which emits irradiation light having a specific wavelength that is absorbed by the silicon film, the temperature is raised to achieve defect repairs or activation of impurities. When an active matrix substrate is heat-treated by the lamp annealing method, since the heating time is short, there are advantages such as no heat damage to the glass substrate. However, in the lamp annealing method, an extreme temperature difference occurs due to a difference in density of an amorphous silicon film on the glass substrate, and significantly large film strains remain. Thus, there may be a serious problem such as cracking of the glass substrate itself. Consequently, the application to the commercial production of low temperature process polysilicon TFTs or the like has not yet been successful.
In the conventional heat treatment apparatuses, since furnaces are heated to high temperatures, strains or deformations easily occur on the surface of a table composed of silica glass. The strains or deformations on the table and heat directly transferred from the table result in deformations such as warpage in substrates placed on the table, which is very disadvantageous. In particular, since non-alkaline glass substrates have poor heat resistance, large warpage or deformation is easily caused at a temperature of approximately 650.degree. C. by the heat transferred from the table, resulting in a significant decrease in non-defective substrate rate.